Area Forecast Discussion
Issued by NWS Sterling, VA

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320 FXUS61 KLWX 130112 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 812 PM EST Sun Nov 12 2017 .SYNOPSIS...
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A weak low pressure system will affect the region tonight into Monday, followed by high pressure on Tuesday and Wednesday. Another front will cross the area Wednesday night into Thursday morning with high pressure moving back in for Friday.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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Surface low pressure along with potent shortwave energy is located over the Ohio Valley this evening. A coastal front remains just off the Mid-Atlantic Coast and weak high pressure continues to wedge in between these two systems. The northern stream system that includes the surface low and shortwave energy over the Ohio Valley will move into our area overnight and it will begin to transfer its energy to a coastal low by Monday morning. This system will be strong enough to back the low-level flow to the southwest...causing warm and moist air to overrun surface colder air in place. The overrunning along with mid-level frontogenetical forcing from the shortwave will cause rain to overspread most areas late this evening into the overnight hours. The most widespread rain is expected near and east of the Blue Ridge and Catoctin Mountains where overrunning will be deepest...but even points to the west should have a period of light precipitation as well. All locations are above freezing this evening and a light north to northeast flow will develop overnight. The airmass upstream to the north contains temps in the upper 30s and lower 40s this evening so it is expected that most areas will remain above freezing overnight. However...dewpoints are in the mid to upper 20s and an in situ cold air damming pattern is likely to setup from wetbulb cooling once rain overspreads the area overnight. This may cause temps to drop close to freezing for a few locations...particularly right near the Mason- Dixon Line into the eastern slopes of the Allegheny Highlands. Pockets of freezing rain are possible toward morning...but confidence is too low for an advisory at this time since most areas are more likely to be above freezing given the fact that temps upstream remain in upper 30s and lower 40s. Another limiting factor is that when temps do get close to freezing...precipitation will be tapering off toward morning as the low-level flow veers back to the west in response to the northern stream system transferring its energy to the coastal low. Another thing to note is rain late this evening and overnight may start out as a mix of rain/sleet due to wetbulb cooling. However...confidence is high that temps will be above freezing during the onset of precipitation so no impacts are expected. Rain totals will range from a few hundredths for most locations west of the Blue Ridge and Catoctin Mountains...to one to two tenths of an inch for locations east of Blue Ridge and Catoctin Mountains...to around 3 tenths of an inch across southern Maryland. Lows tonight in the 30s for most areas.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/... Low will exit the region rapidly on Monday with showers ending across much of the area early Monday morning. After the rain ends, low clouds likely to linger through the morning hours, with breaks of sun by the afternoon. With some northwest flow developing behind the low pressure system, some light upslope rain/snow showers will be possible through the afternoon and evening. Highs will range from the mid 40s to around 50F. High pressure will begin to move into the area Monday night into Tuesday, cresting overhead late Tuesday and Tuesday night. This will lead to dry weather and fair skies. Lows Monday night and Tuesday night in the 30s with highs Tuesday rising into the low 50s. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Two areas of low pressure will pass through the Great Lakes region and into Canada. The first will bring a couple of cold fronts through our region on Thu and again Fri with seasonable temperatures. The second low is stronger and closer, but still NW of us with its track. That second low will also bring a pair of cold fronts through the Mid Atlantic over the weekend. Upslope snow showers are possible over the highlands of the Appalachians as this last system departs with cold flow out of the northwest Saturday night into Sunday. && .AVIATION /01Z MONDAY THROUGH FRIDAY/...
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Light to occasionally moderate rain will overspread the terminals from southwest to northeast late this evening into the overnight...generally between 03z and 07z. Ceilings will lower to MVFR during this time and most likely to IFR levels toward morning as low-level moisture gets trapped underneath the subsidence inversion. Rain will move out of most areas by 12z Monday...but IFR conditions may continue for much of the morning. Cigs will gradually increase and clouds will gradually decrease...but it may take most of the day so MVFR cigs are forecast for much of Monday afternoon before eventually dissipating as drier air moves in from the north. Winds will remain light during this time. VFR will continue through at least Tuesday night with high pressure. Expect mainly VFR conditions Wed thru the weekend. Exception would be in any rain showers. Those are most likely Fri night into Saturday.
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&& .MARINE...
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Light winds will continue through tonight. Winds will increase out of the north/northwest Monday and Monday night behind a departing low pressure system, and winds may approach SCA criteria, but will keep below for now. High pressure returns Tuesday and Wednesday. SCA conditions are possible Thursday behind a cold front.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...MM NEAR TERM...BJL/MM SHORT TERM...MM LONG TERM...CAS AVIATION...BJL/MM/CAS MARINE...BJL/MM/CAS

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